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Publication:
The Chinese Journal of Dental Research

Year 2002
Volume 5 , Issue 3

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Pages: 5 - 24

Thermal Stresses in Ceramometallic Crowns: Firing in Layers

Juergen Lenz, Dr Rer Nat/Matthias Thies, Dr-Ing/Karl Schweizerhof, Dr-Ing/Qiguo Rong, MSc

Objective: This investigation deals with the experimental/numerical determination of transient and residual thermal stresses in ceramometallic crowns, simulating a realistic case of firing in (four) layers. Methods: Axisymmetric (premolar) crown models with a NiCr-frame were equipped with thermocouples on the outer and inner surfaces. The crowns were heated in a porcelain furnace to a homogeneous temperature of 600°C in each simulated firing step. After opening the lid of the furnace, the temperature decreases in the different surface points were measured simultaneously throughout the cooling phase. From these data the temperature and stress distributions in the crown were computed with the aid of the Finite Element Method, assuming linear-thermoelastic behavior of alloy and porcelain and taking into account the dependence of the porcelain’s coefficient of thermal expansion on the number of firings. Results: For the porcelain employed maximum transient thermal stresses in the veneer occurred during the firing of opaque ceramic shortly after opening the furnace. In each further firing step the maximum transient stresses were reduced. Maximum residual stresses were found after the second firing of the body porcelain. Glazing led to considerable relief of the transient thermal stresses. Conclusion: The results allow a deeper understanding of the origins of thermal stresses during the fabrication process of ceramometallic crowns and of the influence of several decisive fabrication parameters on the magnitude of these stresses. In view of the large number of parameters involved, it must be questioned whether a satisfactory definition of “thermal compatibility” of alloy and ceramic can ever be established.

 

 

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